1. Field of the Invention
The present invention relates to a transmission circuit which exhibits high efficiency in a wide dynamic range.
2. Related Art of the Invention
A conventional transmission circuit (for example, see Japanese Patent Publication (Kokai) No. 2002-325109) will be described with reference to FIG. 19. Referring to FIG. 19, data which is generated by a data generation section 1901 is supplied to a quadrature modulator 1902 in which the data is quadrature modulated and frequency converted to a transmission frequency. An output of the quadrature modulator 1902 is supplied to a delta-sigma modulator 1903 in which the output is delta-sigma modulated. The delta-sigma modulated signal is supplied to an amplifier 1904 to be amplified. In the delta-sigma modulator 1903, a signal is converted to a binary or multilevel voltage. Therefore, the nonlinearity of the amplifier 1904 does not cause a problem, and the amplifier can be configured by a high efficiency amplifier. A band-pass filter is connected to an output of the amplifier 1904 to eliminate quantization noise, whereby a desired signal is obtained. The entire disclosure of the above literature is incorporated herein by reference in its entirety.
In the transmission circuit of FIG. 19, when the output power is constant, the characteristics of high efficiency, small distortion, and low noise can be maintained. When the output power is reduced, however, the efficiency, the distortion, or noise characteristics are impaired, so that satisfactory characteristics cannot be maintained over a wide dynamic range.